The present application relates to an optical sheet having 2 pieces of light transmissive films and a display unit including the optical sheet.
In these years, cathode ray tubes (CRT) that were a mainstream of display units in the past have been replaced by liquid crystal display units, since the liquid crystal display units have advantages such as the low electrical power consumption, the space-saving feature, and the low cost.
There are several types of the liquid crystal display units when categorized by, for example, illumination methods in displaying images. As a representative example, a transmissive liquid crystal display unit that displays images by utilizing a light source arranged behind a liquid crystal panel is cited.
In such a display unit, it is extremely important to decrease the electrical power consumption and increase the display luminance in order to increase the commercial value of the display unit. Thus, it has been strongly aspired that the gain of the optical system provided between the liquid crystal panel and the light source is increased, while the electrical power consumption of the light source is kept low as much as possible.
For example, a technique that a prism sheet as a luminance enhancement film is provided between the liquid crystal panel and the light source has been disclosed. In the prism sheet, for example, a plurality of prisms in the shape of an isosceles triangle pole having an apex angle of 90 degrees are arranged in parallel on a resin film. The front luminance is improved by utilizing the light-focusing effect of the prisms. Further, in International Publication No. WO2006/071616, a technique that a prism sheet having the polarization split function is provided between the liquid crystal panel and the light source has been disclosed. In such a prism sheet, in addition to the light-focusing effect of the prisms, polarization split is performed on the inclined plane of the prisms by utilizing interface reflection due to the difference of critical angles, and thereby the front luminance is improved.
However, in the foregoing prism sheet, though the front luminance is improved, the luminance view angle that is called dark band, cutoff or the like is rapidly lowered. Thus, to avoid the rapid lowering of the luminance view angle, for example, one of the various diffusion sheets, which is publicly known, may be provided between the liquid crystal panel and the polarization split device. However, in this case, there is an issue that the front luminance is lowered by insertion of the diffusion sheet.